1. Technical Field
The present invention relates to a method of making seals including a sealing lip of low friction material which is adapted to engage a relatively moveable part. More specifically, the invention relates to a method of molding an oil seal wherein a wafer of low friction material and a seal case are clamped against a portion of a mold by rings of elastomeric material while the elastomeric material is molded to form the body of the oil, or shaft, seal.
2. Prior Art
Oil seals are used in many different types of machinery to seal lubricated areas of relatively moveable parts. A high quality oil seal should exert a minimum of friction between moveable parts and must be capable of forming a tight seal even after extended periods of use. Polytetrafluoroethylene (PFTE) or other low friction material is sometimes used at the sealing lip of a seal for long wear and its low friction characteristics. However, since PTFE is a stiff material it is often desireable to use it in combination with an elastomeric backing material which is flexible enough to compensate for some misalignment of the seal or shaft runout.
A shaft seal is a frequently used type of oil seal which is installed in a bore to seal against a rotating shaft. Shaft seals having a sealing lip of low friction material secured to a rigid case by means of an elastomeric material are precision parts manufactured to close tolerances. The sealing lip and rigid case must be accurately located to assure satisfactory seal performance.
Holding a wafer of low friction material in a mold as it is formed during the molding operation has always been a problem in the manufacture of such a seal. Similarly, establishing the location of the rigid case within the mold can be difficult when the outer diameter of the case is to be covered with elastomeric material during the same molding process.
Various methods of molding shaft seals having a lip made of low friction material are known in the prior art. However, prior art methods frequently fail to locate the wafer of low friction material precisely relative to the case, especially when the seal case includes an elastomeric outer diameter.
One such prior art method is disclosed in U.S. Pat. No. 4,171,561 to Bainard wherein a wafer of low friction material is secured to a rigid metal case by a molded elastomeric material. In the Bainard seal, the metal case is not positively located during the molding operation. Consequently, the metal case may be lifted by the elastomeric material during the molding operation thereby preventing the metal case from being precisely located relative to the sealing lip in the finished product.
Another prior art method of molding a shaft seal having a low friction lip is disclosed in U.S. Pat. No. 4,159,298 to Bainard in which a transfer mold is used to make a seal having an annular metal case and a polytetrafluoroethylene (PTFE) sealing element bonded together with elastomeric material. The metal case is held in place during the molding operation by a plurality of support pins that may become worn or distorted in time, resulting in the expense of frequent inspection and replacement. The pins also create bare spots on the metal case which may be objectionable under certain circumstances. In addition, the annular PTFE wafer is not held in place during the molding operation, making it subject to migration during molding.
A prior art method of molding a seal having an elastomer coating on the outer diameter is disclosed in U.S. Pat. No. 4,006,210 to Denton. However, the Denton molding apparatus and process yields a seal in which the location of the seal lip and case are subject to substantial variation in location relative to one another. In the Denton seal design the PTFE wafer is held directly against the metal case without intermediate elastomer for providing adequate flexibility to compensate for shaft run out or variation in the location of the seal lip relative to the metal case.
In summary, none of the methods shown in the above patents positively preclude migration or mislocation of the metal case or PTFE wafer. It is to this end that the present invention is directed.